This invention relates to an apparatus for the control of an internal combustion engine and, more particularly, to an apparatus for detecting revolutions of the engine.
In recent years, much effort has been made for decreasing the emission of pollutants. Unfortunately, countermeasures against the emission of exhaust gases have, in many cases, resulted in lowering the conversion efficiency of heat energy, released by the combustion of fuel, into mechanical energy, so that larger amounts of fuel have become necessary for automobile operation.
In order to attain a high conversion efficiency from heat energy to mechanical energy and the low emission of pollutants at the same time, electronic controls have been developed using a digital processing unit.
In such a digital engine control, the number of revolutions of the engine is important input data together with the flow rate of air supplied into a combustion chamber. In other words, the extent of the precision of such input data has a large amount of influence on the engine control performance. The number of revolutions of the engine ranges from less than 100 r.p.m. during low speed cranking to as much as 6000 r.p.m. or more at high speeds. The revolutions N can be obtained by counting, during a certain period of time, pulses generated by a crank angle sensor. It is now assumed the crank angle sensor is designed to generate a pulse each time the engine rotates 0.5 degrees of the crank angle and that the measurement time width is a fixed value TW.sub.o. It is also supposed that the number of revolutions to be measured is in the range of 0 through 6400 r.p.m. since revolutions of more than about 6000 r.p.m. are generally considered to lie in a dangerous region. If the measurement data is indicated as a digital signal of 2.sup.10 bits, then the time duration TW.sub.o (m sec) is given by the following equation. ##EQU1## Thus, if P angle pulses are sensed in the time duration of about 13 (m sec), the number N of revolutions is given by the following equation. EQU N=25/4P(r.p.m.)
The resolution of the revolution number input value in this case is 25/4 (r.p.m.) per digit. Since an error of .+-.1 digit at the maximum can exist in the measured value, the relative error (%) to the number N of revolutions is expressed as follows. EQU .epsilon.=25/4N.times.100(%)
As a result, as N becomes smaller, the error increases sharply. As is well known, the fuel injection time T.sub.p is expressed in a manner to vary in dependence upon the number N of the engine revolutions. Therefore, it is very important for the precision of engine control to accurately measure the engine revolutions.